Combining the radar, satellite, lightning positioning system data, etc, the characteristics and evolution of cloudtoground lightning and the electric field of the Mesoscale Convective System (MCS) in Nanjing on 5 June 2009 are analyzed. Positive cloudtoground lightning played a dominant role in the developing stage of MCS; negative cloudtoground flashes are dominant in the mature stage; positive flashes are relatively frequent over the stratiform cloud areas in the extinction stage. This process of the electric field evolution can be divided into four stages, and the evolution and characteristics of the electric field in convective areas are analyzed. The electric field of MCS over the convective area is simulated with the quasinormal distribution model. It is found that the simulated electric field accords with observation, and the quasinormal distribution model can response to slight changes in the electric field.